1. Field of the Invention
The present invention relates generally to position regulation in printing systems. More particularly, the present invention relates to a control system for a printing press that regulates relative positions of drive units within the press.
2. State of the Art
Because conventional web-fed printing presses such as those used for printing newspapers, have traditionally not been expected to produce high quality or high resolution print, a relatively high degree of tolerance for print quality degradation has existed in the printing industry. However, increasing desire for enhanced quality and resolution of printed products has generated a need for printing presses capable of providing printed products having a high level of quality and resolution. The production of higher quality printed products typically involves an attendant reduction in printing speed. Despite this, there is an increased desire within the printing industry for printing presses capable of operating at high speeds. Simultaneously satisfying the needs for quality, resolution, and speed has proven to be a very difficult task.
A conventional printing press typically includes a number of printing units. The relative positions of drive shafts of the printing units must be accurately controlled to maintain proper registration of the different printing units. Proper registration of the different printing units prevents errors such as printing registration errors, web tension errors, web-to-web registration errors, and/or signature cutoff errors. Such errors are aggravated as printing speeds increase.
In certain printing presses, each group of printing units has a drive unit including a drive shaft which is connected with, and driven by, an output shaft of an electric motor for that particular group. A speed controller provides a speed control signal to control the rotational speed of the output shaft of the electric motor. Other groups of printing units as well as non-printing stages within the press can also have drive units. Typically, one of the drive units of the press is designated as a "master", and receives a signal indicating a desired speed for a web of paper passing through the printing press. This desired speed signal is provided to the speed controller of the master to control the speed of the master's drive shaft. Signals representing the actual speed and position of the master's drive shaft are transmitted to the other drive units, designated as "slave" units. The speed controller of each slave unit provides a speed control signal to the slave unit's electric motor based on the actual positions of the slave unit's drive shaft and the master's drive shaft, to cause the slave unit's drive shaft to track both the speed and position of the master unit drive shaft. Ideally, the drive shaft of each slave unit has the same position and speed as the master's drive shaft.
Many types of regulators are available for regulating the position of a slave drive shaft relative to the position of a master drive shaft, such as phase-locked loop regulators and synchro regulators. A synchro regulator typically includes a synchro to transform the angular position of the slave drive shaft into an electrical output signal. The position of the slave drive shaft relative to the position of the master drive shaft is then regulated in response to the electrical output signal from the synchro regulator. regulators having open-loop compensation, such as "forcing", "speed reference", and "dp/dt feed-forward ", are also known. In addition, a "type-3" regulator that doubleintegrates a position error signal (i.e., a difference between the slave and master drive shaft positions) can also be used. Such a regulator is described in U.S. Pat. No. 5,049,798, issued Sep. 17, 1991, which is incorporated by reference into this document.
FIG. 1 illustrates a conventional printing press 10 including infeeds 12 and 14, a group 207 of print units 200-206 and a group 23 of print units 16-22, a dryer 24, chill units 25 and 26, and folder units 28 and 30. Each of the print unit groups 207 and 23 and the folder units 28 and 30 has a drive unit. A master reference signal source 32 provides a signal indicating a desired printing press speed to the drive unit of the group 207, i.e., a speed command signal. The drive unit of the group 207 is designated as a master drive unit. The other drive units corresponding to the group 23 and the folder units 28 and 30 are designated as slave print units, and track the position and speed of the master drive unit.
FIG. 2 shows details regarding internal components of the drive units within the groups 207 and 23 and the folder units 28 and 30, and connections among the drive units and the master reference signal source 32. In particular, the speed command signal from the master reference signal source 32 is input to a speed control 210 of the master drive unit in the group 207 to control a speed of the motor 260. The speed command signal can be an analog signal or a digital signal. A position encoder 230 determines an actual position of a drive shaft 240 driven by the master drive unit motor 260. Since position information provided by the position encoder 230 can be used to derive speed information, the position encoder 230 can optionally provide feedback to the speed control 210 to ensure that the actual speed of the master drive unit drive shaft 240 matches the desired speed.
The speeds and positions of the slave drive unit drive shafts 242-246 are controlled to match the speed and position of the master drive unit drive shaft 240 using the speed of the master drive unit drive shaft 240 together with feedback regarding positions of the slave drive unit drive shafts 242-246 relative to the position of the master drive unit drive shaft 240.
As shown in FIG. 2, the slave drive units have motors 262-266 that drive drive shafts 242-246. Position encoders 232-236 determine actual positions of the drive shafts 242-246 and provide corresponding feedback signals to regulators 222-226 that indicate the determined positions. As noted above with respect to the position encoder 230 of the master drive unit, information provided by the position encoders 232-236 can be used to determine both speeds and positions of the corresponding drive shafts 242-246. The output signal from the position encoder 230, which indicates the actual position of the master drive unit drive shaft 240, is provided as a position reference signal to the regulators 222-226 of the slave drive units within the print unit group 23 and the folder units 28 and 30 as shown in FIG. 2. The regulators 222-226 compare the master drive unit position encoder 230 output signal with the outputs of the position encoders 232-236, and based on the comparison provide command signals to the speed controls 212-216 to control the speed of the motors 262-266 so that the slave drive unit drive shafts 242-246 track the speed and position of the master drive unit drive shaft 240.
According to one configuration of the position encoder 230, the position encoder 230 provides a pulse for each angular increment through which the master drive unit drive shaft 240 rotates. Thus, as the drive shaft 240 rotates, the position encoder 230 will output a stream of pulses. A number of pulses output by the position encoder 230 during a time interval indicates how much the drive shaft 240 changes position during the time interval. An average speed during the time interval can be easily determined by dividing the change in position by the duration of the time interval.
The angular increment corresponding to a pulse is specified so that the position encoder 230 will output 2,048 pulses during each complete revolution. The position encoder 230 is monitored and the pulses output by the position encoder 230 are counted by a counter (not shown). The counter typically turns over upon completion of one revolution, i.e., resets to zero after counting up to 2,048. Some implementations use a different number of pulses per revolution, and other implementations reset the counter less frequently than every revolution. The position encoders 232-236 have a configuration similar to the configuration of the position encoder 230. Positions of the encoders 230-236 can be synchronized by simultaneously resetting the corresponding counters to zero, for example when the paper web is moving through the printing press at a slow and steady velocity. Thereafter, any difference in values of the counters indicates a phase or position difference. For example, if at a certain point in time the value of the counter corresponding to the master drive unit position encoder 230 is 1,000 and the value of the counter corresponding to the slave drive unit position encoder 232 is 795, then at that point in time the position of the slave drive unit drive shaft 242 is lagging the position of the master drive unit drive shaft 240 by 205 angular increments, or about 36.degree.. Computer software accurately tracks phase differences as the counters turn over, and can also track phase differences greater than one complete revolution. In the printing press shown in FIG. 2, each of the regulators 222-226 has a counter (not shown) for counting pulses from the master drive unit position encoder 230, and a counter (not shown) for counting pulses from one of the slave drive unit position encoders 232-236. In some configurations the counters are located within or near the corresponding position encoders.
Although the regulators 222-226 are configured to synchronize slave drive unit drive shaft speeds and positions with those of the master drive unit drive shaft 240, they do not address problems that arise when mechanical disturbances or control errors occur at the master drive unit drive shaft 240. These errors are transmitted to the slave units, which try to emulate the errors. Thus, an error at the master drive shaft can "ripple" down to the slave units. If the disturbances are large, regulator performance is disrupted or compromised, and process problems such as misregistration can result.
Events that can cause speed and position disturbances during printing operations include, for example, a "blanket wash". When a blanket wash is performed, accumulated dirt and lint are washed or brushed from a blanket roller in the printing press. When a blanket wash is performed at the master drive unit, disturbances can occur in the speed and position of the master drive unit drive shaft 240. These disturbances are transmitted to the slave units, potentially disrupting smooth operation of the printing press and resulting in paper waste and reduced print quality. Other printing press operations can also cause disturbances. For example, disturbances can occur when a new paper web is spliced onto an existing web, or when the web is cut at the folder stage.
Because errors at a master unit are transmitted to the slave units, a unit of the printing press where errors occur least frequently and with lowest magnitudes is usually chosen to be a master unit. For example, in a printing press that has infeed units, printing units, drying units, chill roll units and folder units, as shown for example in FIG. 1, one of the printing units is typically chosen to be a master unit instead of one of the folder units. This is because the cutting operation at the folder unit produces a much larger transient disturbance than the blanket wash operation at the printing unit.
Although human operators try to run printing presses as smoothly as possible to minimize transient disturbances that arise at the master drive unit, disturbances do occur. When disturbances occur, filtering networks and reference dead bands are used until the disturbances are identified and corrected. Paper waste and reduced print quality resulting from the disturbances are accepted as a matter of course. However, it would be desirable to provide a printing press wherein effects of transient disturbances are reduced or eliminated so that improved print quality and reduced waste can be achieved, even at higher press speeds.